The invention relates to a heating unit utilizing a two-phase liquid heat transfer system in a boiler heat exchanger structure to provide greater efficiency and thereby save energy. More specifically, the heating unit has a boiler chamber in which the lower phase liquid is vaporized and percolated through the upper phase liquid into a radiator where heat transfer to an airflow thereacross occurs. Thereafter the vapor continues its circulation by entering a header where the vapor is condensed back to the liquid state and returned to the boiler chamber to repeat the cycle.
There are available today a large number of differing designs for heating units of varying efficiencies, most of which can be grouped into three major areas of design concept, mainly the hot water or steam circulation type, the forced air type, and the newer heat pump type.
The hot water or steam circulation type probably is the most efficient type available today in over all performance. The most popular is the hot water unit where the water is heated and circulated by an electric pump to base board radiators or radiant floor panels in the rooms of the structure to be heated. Some of the drawbacks of this type of system are: a more expensive and complicated initial installation mechanically and electrically due to the added piping, insulation therefor, electric pump and controls therefor, and the more numerous thermostatic controlled electric valves on the lines leading to various portions of the structure to be heated; because of the more complicated design, more maintenance to keep the system in operation; a lack of air circulation likely to cause stagnation; the noise nuisance of water bumping as the system is fired up; and a lack of ability to control the humidity of the air because these systems are closed. Because of its economics, this type of heating system finds wide acceptance in commercial construction today. Prior art directed toward boiler designs in this type of heating system which are of interest include U.S. Pat. Nos. 1,898,571; 2,154,021; 2,277,094; 2,508,736; 2,820,134; and, 3,210,005. Although the forgoing patents are addressed to the problems attendant the use of a boiler, none have achieved the results of the present invention.
The forced air type is probably the most popular type for the residential market. Air flow is directed across heating elements or a convector to heat the air to a point about 30.degree. above the room temperature and then piped through air ducts to the rooms to be heated. Cold air intake registers supply this flow such that a complete air cycle is set up in the structure to be heated. This type of heating system has the advantage of being: simpler to install and maintain, usually cheaper to install; able to provide an easy way to control humidity in the structure; easily used for both heating in winter and cooling in summer with less alteration of equipment; able to provide good air circulation to prevent stagnation; and are generally quieter in operation. The major drawback of this type of heating system is its lower efficiency.
The newer heat pump type is gaining more acceptance in more temperate climates because it employs the advantages of a forced air system at a higher efficiency within a given temperature range. The heat pump is basically an air conditioner and is sized according to air conditioning capacity necessary for the given structure. It has a reversing valve to convert it to a heating unit. It operates as an air conditioner in that it employs two coils, one on each side of a compressor. The low pressure side coil absorbs heat which is emitted from the high pressure side coil. Air flow is directed across both coils to affect the heat transfer. The reversing valve is operated by a thermostat detecting outside temperature and controls which side is the high pressure coil. Also for winter operation a defrosting system is necessary on the outside coil because during the heating cycle, frost will deposit on the outside coil if the temperature of the coil falls below 32.degree. F. If ice is allowed to build up the efficiency of the heat pump is significantly reduced. One defrost system directs hot gas from the compressor discharge to the outside coil long enough to melt any ice formation thereon. During this defrost cycle though, no heat is produced in the inside coil to heat the structure. This means that when the outside temperature falls much below 32.degree. F., supplementary electric resistance heaters will be required, thereby reducing the efficiency as the temperature decreases. At peak efficiency conditions the heat pump can produce 8,500 btu's per kilowatt hour of electricity for a Coefficient of Performance of 2.489 based upon 1 kilowatt hour of electricity producing 3,414 btu's, the standard conversion factor. The major drawback of this type of heating system then is the narrow temperature range at which it maintains a high efficiency. Also, due to the fairly complex nature of this equipment, maintenance costs are higher and the high pressures involved can present a safety hazard.